1. Field of the Invention
This invention relates generally to a catalyst and method employing such catalyst for purifying crude terephthalic acid, crude isophthalic acid or a crude naphthalene dicarboxylic acid, and more particularly concerns the use in the aforesaid purification method of a catalyst comprising at least one metal of Group VIII of the Periodic Table supported on a carrier comprising titanium dioxide.
2. Discussion of the Prior Art
Polymer grade or "purified" terephthalic acid and isophthalic acid are the staring materials for polyethylene terephthalates and isophthalates, respectively, which are the principal polymers employed in the manufacture of polyester fibers, polyester films, and resins for bottles and like containers. Similarly, polymer grade or "purified" naphthalene dicarboxylic acids, especially 2,6-naphthalene dicarboxylic acid, are the starting materials for polyethylene naphthalates, which can also be employed in the manufacture of fibers, films and resins. A purified terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid can be derived from a relatively less pure, technical grade or "crude" terephthalic acid, isophthalic acid or "crude" naphthalene dicarboxylic acid, respectively, by purification of the crude acid utilizing hydrogen and a noble metal catalyst, as described for terephthalic acid in U.S. Pat. No. 3,584,039 to Meyer. In the purification process, the impure terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid is dissolved in water or other suitable solvent or solvent mixture at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst, which conventionally is palladium on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915. This hydrogenation step converts the various color bodies present in the relatively impure phthalic acid or naphthalene dicarboxylic acid to colorless products. Another related purification-by-hydrogenation process for aromatic polycarboxylic acids produced by liquid phase catalyst oxidation of polyalkyl aromatic hydrocarbons is described in Stech et al., U.S. Pat. No. 4,405,809.
Carbon is conventionally used as the support material for the noble metal in the catalyst employed in the aforesaid purification method. A common disadvantage of the use of a carbon support is that carbon fines are often generated during commercial operations. The generation of such fines can be minimized but generally cannot be completely avoided. During the subsequent esterification process, such particulates introduced with the particular purified acid, for example, terephthalic acid, isophthalic acid or 2,6-naphthalene dicarboxylic acid, can plug filters and thereby cause interruptions in the process. Other such particulates that bypass the filter may be incorporated into the resulting polyester fiber or film and cause fiber breakage or film distortion.
For this reason, it is highly desirable to use other materials as the support material in the catalyst employed in the aforesaid purification method. However, because of the highly corrosive conditions under which the aforesaid purification is performed, it has proven difficult to develop suitable non-carbon catalyst supports for use in the purification catalyst. For example, as indicated in Meyer, U.S. Pat. No. 3,584,039 in column 5, lines 10-14, hot aqueous solutions of terephthalic acid dissolve supporting materials such as natural and synthetic alumina, silica, silica-alumina, kieselguhr, calcined clays, zirconium supports and other metal oxides and metal salt containing supports.
M. Bankmann, R. Brand, B. H. Engler and J. Ohmer, "Forming of High Surface Area TiO.sub.2 to Catalyst Supports," Catalysis Today, Vol. 14, pages 225-242 (1992), contains an extensive discussion of the use of titanium dioxide having a high surface area as a catalyst support. The article (which was previously presented in a substantially identical form by R. Brand at the Fall, 1991, American Chemical Society meeting) indicates that the titanium dioxide must have a high surface area in order to be a suitable catalyst support and discusses only titanium dioxide having surface areas of 50 and 100 square meters per gram. The article discusses the extrusion process for manufacturing titanium dioxide having the requisite high surface area and the effect of the raw materials, additives and process parameters employed in the extrusion process on catalytically important characteristics of the resulting titanium dioxide. As disclosed, the extrusion process involves the steps of (1) mixing and kneading the raw materials, (2) extruding, (3) drying, and (4) calcining, each of which influences the quality of the resulting support. Correlations between the concentration of water, plasticizers and binders and the type of titanium dioxide raw material employed in the mixing and kneading step and the crushing strength, attrition resistance, pore diameter and pore volume of the resulting catalyst support, and correlations between the calcination temperature and the surface area, pore volume, mean pore diameter and pore size distribution and the degree of transformation from the anatase crystalline phase to the rutlie crystalline phase in the resulting catalyst support, are discussed in the article. More particularly, the use of catalysts containing palladium, platinum or rhodium components supported on titanium dioxide for selective hydrogenation is discussed. On pages 240-241, the use of such catalysts to hydrogenate a para-substituted benzaldyhyde to the corresponding para-substituted benzyl alcohol or para-substituted toluene is disclosed. The table on page 241 indicates that the para-substituent can be a carboxylic acid group, a methyl group or a halogen. The article discloses that the results of the hydrogenation of para-substituted benzaldyhyde were substantially different depending upon whether the catalyst contained palladium, platinum or rhodium on the titanium dioxide support. The article indicates that the titanium dioxide must have a high surface area in order to be a suitable catalyst support and discusses only titanium dioxide having surface areas of 50 and 100 square meters per gram. In addition, the article discloses that depending on the reaction temperature employed, the reduction of a parasubstituted benzaldehyde affords either of several products with high selectivity and in high yield. Except for the catalyst, the reaction temperature and the hydrogen pressure employed, the article does not disclose the conditions under which the hydrogenation was performed.
Schroeder et al., U.S. Pat. No. 4,743,577, discloses that the use of catalysts containing palladium finely dispersed on carbon in the aforesaid purification of terephthalic acid derived from the oxidation of p-xylene results in contamination of the resulting purified terephthalic acid with fines produced by abrasion of the carbon granulates due to their relatively low crush strength and abrasion resistance. This patent discloses that reduced fines contamination results from the use instead of a catalyst containing a thin layer of palladium, nickel, rhodium, platinum, copper, rhuthenium and cobalt on a porous sintered support of metallic titanium, zirconlure, tungsten, chromium, nickel and alloys incorporating one or more of these metals. The surface area of palladium-plated supports of titanium, incoriel and nickel are disclosed as 0.22, 0.55 and 1.21 square meters per gram, respectively, which are very significantly smaller than specific surface area of a palladium on active carbon catalyst.
Sikkenga et al., pending U.S. patent application Ser. No. 07/900,593, filed Jun. 18, 1992, discloses the preparation of an aromatic carboxylic acid by the liquid phase catalyzed oxidation of an alkyl-substituted aromatic compound such as o-, m-, or p-xylene or 2,6-dimethylnaphthalene. The application further discloses on page 11, lines 23-31, that the resulting aromatic carboxylic acids can be purified by hydrogenation thereof in the presence of a catalyst comprising one or more Group VIII metals deposited on a support such as alumina, titania or carbon. The application contains no other mention of titania.
Holzhauer et al., pending U.S. patent application Ser. No. 07/900,637, filed Jun. 18, 1992, discloses on page 27, lines 1-12, a method for purifying 2,6-naphthalene dicarboxylic acid by treating it with hydrogen in the presence of a hydrogenation catalyst containing one or more of platinum, palladium, rhodium, ruthenium, osmium and iridium supported on alumina, silica-alumina, silica, titania, clays and zirconia. The application contains no other mention of titania.
Timruer et al., U.S. Pat. No. 4,831,008, discloses the use of a catalyst containing a rhodium-containing component supported on titanium dioxide for the hydrogenation of benzene, toluene, o-xylene, terephthalic acid, disodium terephthalate, and diethyl terephthalate, in which the aromatic ring is hydrogenareal.